546 GASTEROPODA. 



particular strictly resemble each other, unless perhaps in size, pour the 

 secretion that they furnish into three different situations, the first into 

 the oesophagus; the second into the oesophagus likewise, and the third 

 into the gizzard, which forms the first of three stomachal cavities. 



(1455.) In Doris, a figure of which is given above, a still more extra- 

 ordinary arrangement is met with. One set of ducts derived from the 

 liver penetrate the stomach, and pour the bile into that cavity, while 

 another large canal, equally given off from the liver, terminates at the 

 exterior of the body by an orifice situated in the vicinity of the anus 

 (fig. 266) ; and thus a part of the bile secreted would seem to be 

 expelled from the system as excrementitious matter a fact of no 

 ordinary importance to the physiologist, as it would itself go far to prove 

 that the function of the liver is not merely limited to the supply of a 

 secretion of importance in the digestion of food, but that it powerfully 

 cooperates with the respiratory system in purifying the circulating 

 fluids by decarbonizing the blood. 



(1456.) Other secretions, apparently of an excrementitious character, 

 are furnished by many Gasteropods. Thus, in Aplysla a glandular 

 mass is imbedded in the opercular flap that protects the gills, from 

 which, at the pleasure of the animal, a reddish liquor is made to exude 

 in sufficient abundance to obscure the water around it, and thus conceal 

 it from pursuit. Another gland furnishes an acrid limpid fluid, that 

 distils from an orifice near the oviduct ; but the use of this last secretion 

 is as yet unknown. 



{1457.) The scattered condition of the nervous ganglia, characteristic 

 of the HETEEOGANGLIATA, is well exhibited in the Pectinibranchiate 

 Gasteropods, more especially as it not unfrequently happens that the 

 ganglionic centres themselves are of an orange or reddish colour, while 

 the nerves derived from them present their usual appearance. 



(1458.) In Bucdnum the brain still occupies its usual position above 

 the oesophagus (fig. 275, d), and gives off nerves to the organs of sensa- 

 tion, and large twigs (c c) to the eminently sensitive proboscis. A large 

 nervous mass placed beneath the oesophagus (i) is connected with the 

 former by several communicating nerves that embrace the oesophageal 

 tube. Other ganglia, of smaller size (k, I, n), are distributed in distant 

 parts of the body, and supply the viscera to which they are contiguous, 

 whilst they are connected among themselves, and with the brain, by 

 nervous cords passing from one to another. 



(1459.) In Pterotrachea the same dispersion of the central ganglia of 

 the nervous system is equally evident. The brain and nervous collar 

 around the oesophagus occupy their usual situation, and give nerves to 

 the tentacles, eyes, and parts around the mouth; while four smaller 

 ganglia (fig. 269, ?') are placed in the immediate vicinity of the foot, to 

 which and to the neighbouring viscera they distribute their branches. 



(1460.) But in the most elevated Gasteropods the ganglia assume 



